Add -stdlib=libc++ to {CXX,LD}FLAGS and not CXX

This allows things work even if a non-default of CXX is specified on the
command line.

configure

@@ -20415,7 +20415,8 @@ if test "x$wxUSE_MACOSX_VERSION_MIN" != "x"; then
 fi
 
 if test "$HAVE_CXX11" = "1" ; then
-            CXX="$CXX -stdlib=libc++"
+            CXXFLAGS="$CXXFLAGS -stdlib=libc++"
+    LDFLAGS="$LDFLAGS -stdlib=libc++"
 fi
 
 if test "x$retest_macosx_linking" = "xyes"; then

configure.in

@@ -1233,8 +1233,9 @@ fi
 
 if test "$HAVE_CXX11" = "1" ; then
     dnl We also need to use libc++ standard library instead of libstdc++ for
-    dnl C++11 support.
-    CXX="$CXX -stdlib=libc++"
+    dnl C++11 support, both when compiling and when linking.
+    CXXFLAGS="$CXXFLAGS -stdlib=libc++"
+    LDFLAGS="$LDFLAGS -stdlib=libc++"
 fi
 
 dnl If either an SDK or a version option was added, make sure that we can